Olfaction

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Olfaction

One of the chemical senses, specifically the sense of smell. Olfaction registers chemical information in organisms ranging from insects to humans, including marine organisms. For terrestrial animals, its stimuli comprise airborne molecules. The typical stimulus is an organic chemical with molecular weight below 300 daltons. A few inorganic chemicals can also stimulate olfaction, notably hydrogen sulfide, ozone, ammonia, and the halogens.

The anatomy of olfactory structures and the neurophysiology of olfaction differ significantly among different animal groups. For examples, insect olfactory receptors exist within sensory hairs on the antennae. The olfactory organ of fishes resides typically in tubular chambers on either side of the mouth. In terrestrial vertebrates, the olfactory receptors reside within a sac or cavity more or less similar to the human nasal cavity. The olfactory mucosa patch in the cavity characteristically contains millions of receptor cells, though in some olfactory-dominated mammals, such as the dog and rabbit, it contains tens of millions. The location of the olfactory mucosa relative to air currents in the cavity plays some role in the ongoing olfactory vigilance of the organism. In the human the mucosa sits out of the main airstream. During quiet breathing eddy currents may carry just enough stimulus to evoke a sensation, whereupon sniffing will occur. Sniffing amplifies the amount of stimulus reaching the receptors by as much as tenfold.

Reception of the chemical stimulus and transduction into a neural signal apparently occur on the olfactory receptor cilia. The ciliary membrane contains receptor protein molecules that interact with stimulating molecules through reversible binding. Vertebrate receptor cells show broad tuning, that is, they respond to many odorants.

Adjacent points in the mucosa generally project to adjacent points in the olfactory bulb of the brain (see illustration). The synapses between the incoming olfactory nerve fibers and the second-order cells, mitral cells, occur in basketlike structures called glomeruli. On average, a glomerulus receives about 1000 receptor cell fibers for each mitral cell. The location of cells within the bulb seems to play a role in encoding odor quality: each odorant stimulates a more or less unique spatial array.

Location of the olfactory bulbs at the interior surface of the human brain and their connections via the anterior commissureenlarge picture
Location of the olfactory bulbs at the interior surface of the human brain and their connections via the anterior commissure

The central neural pathways of the olfactory system have a complexity unmatched among the sensory systems. One pathway carries information to the pyriform cortex (paleocortex of the temporal lobe), to a sensory relay in the thalamus (dorsomedial nucleus), and to the frontal cortex (orbitofrontal region). This pathway seems rather strictly sensory. Another pathway carries information to the pyriform cortex, the hypothalamus, and other structures of the limbic system. The latter have much to do with the control of emotions, feeding, and sex. The strong affective and motivational consequences of olfactory stimulation seem compatible with projections to the limbic system and with the role of olfaction in certain types of physiological regulation. In many vertebrate species, reception of pheromones occurs via an important accessory olfactory organ, known as the vomeronasal organ, which characteristically resides in the hard palate of the mouth or floor of the nasal cavity. See Pheromone

Human olfactory sensitivity varies from odorant to odorant over several orders of magnitude. A common range of thresholds for materials used in fragrances and flavors is 1 to 100 parts per 109 parts of air. Thresholds gathered from various groups of human subjects permit certain generalities about how the state of the organism affects olfaction. For instance, persons aged 70 and above are about tenfold less sensitive than young adults. Males and females have about equal sensitivity, except perhaps in old age, where females are more sensitive. Persons with certain medical disorders, such as multiple sclerosis, Parkinson's disease, paranasal sinus disease, Kallmann's syndrome, and olfactory tumors, exhibit decreased sensitivity (hyposmia) or complete absence of sensitivity (anosmia).

Above its threshold, the perceived magnitude of an odor changes by relatively small amounts as concentration increases. A tenfold increment in concentration will cause, on average, about a twofold change in perceived magnitude. The perceived magnitude of an odor is often greatly influenced by olfactory adaptation, a process whereby during continuous short-term exposure to a stimulus its perceived magnitude falls to about one-third of its initial value.

The stimuli for olfaction are commonly complex, that is, they are mixtures. Such products as coffee, wine, cigarettes, and perfumes contain at least hundreds of odor-relevant constituents. Only rarely does the distinctive quality of a natural product, such as a vegetable, arise from only a single constituent. A chemical analysis of most products will not usually allow a simple prediction of odor intensity or quality. One general rule, however, is that the perceived intensity of the mixture falls well below the sum of the intensities of the unmixed components.

General notions about the properties that endow a molecule with its quality have spawned more than two dozen theories of olfaction, including various chemical and vibrational theories. Most modern theories hold that the key to quality lies in the size and shape of molecules, with some influence of chemical functionality. For molecules below about 100 daltons, functional group has obvious importance: for example, thiols smell skunky, esters fruity, amines fishy-uriny, and carboxylic acids rancid. For larger molecules, the size and shape of the molecule seem more important. Shape detection is subtle enough to enable easy discrimination of some optical isomers. Progressive changes in molecular architecture along one or another dimension often lead to large changes in odor quality. No current theory makes testable predictions about such changes. See Chemical senses, Chemoreception

Olfaction

 

the sense by which the odor of chemical compounds in the environment are perceived in specialized organs of humans and animals.

Olfaction is one of the forms of chemoreception. The chemical compounds to which olfaction is sensitive are odorous substances that are present in the environment in low concentrations and are, as a rule, in themselves neither beneficial nor harmful to the organism. These compounds merely act as signals that indicate the existence of certain objects or conditions in the environment. In terrestrial environments, odorous substances are vapors that are transported to the olfactory receptors in a current of air or by diffusion. In an aquatic environment, the term “odorous substances” can also refer to nonvolatile compounds, even though such compounds lack odor in the ordinary sense. For example, nonvolatile solutions of certain amino acids excite the olfactory receptors of fish.

The role of olfaction and the degree of development of the olfactory organs differ greatly among the various animal species. Mammals are divided into macrosmatics, with highly developed olfaction; microsmatics, with relatively poor olfaction; and anos-matics, which lack typical olfactory organs. Most mammals are macrosmatics. Microsmatics include seals, baleen whales, and primates. Anosmatics include the toothed whales. Animals rely on olfaction to search for and select food and to detect prey and predators. Bioorientation, marking of territorial boundaries, and mating are other functions that are dependent on olfaction (seeBIOORIENTATION).

Pheromones constitute a special group of odorous substances. These are compounds that are secreted by animals, usually from specialized glands, into the environment in order to regulate the behavior of other members of the same species. Pheromones include many substances that can act as identifying labels, at-tractants, or warning signals. As a rule, pheromones in vertebrates act in combination with visual, auditory, and tactile signals. In insects, on the other hand, behavior can be determined exclusively by the action of a pheromone.

In humans the role of olfaction is relatively modest, although not altogether unimportant. A higher incidence of food poisoning is found among persons with olfactory disorders, since the taste of food is in large measure determined by olfactory sensations. Olfactometers are instruments that measure the intensity and duration of the effects of an odorous substance. The olfactory threshold, that is, the minimum concentration at which the odor of a substance can be perceived, has been determined for many substances. The olfactory receptors are highly sensitive to certain odorous substances. For example, a human perceives the presence of trinitrobutyltoluol at a concentration of about 5 × 10-15 g, or 10 million molecules, per cm3 of air. A dog senses butyric acid at a concentration of 10,000 molecules per cm3 of air, while the male silkworm moth perceives the presence of bombykol—the female sex pheromone—at a concentration of 100 molecules per cm3. In these three cases of highly sensitive chemoreception, apparently a single molecule of the odorous substance is sufficient to excite an individual olfactory receptor.

With prolonged or repeated contact with an odorous substance, the olfactory threshold to that substance and, to a lesser degree, to other odorous substances is increased. This attenuation of olfactory sensitivity is partially due to fatigue in the olfactory receptors and partially due to changes in the brain centers of the olfactory analyzer. In addition to exciting olfactory receptors, many odorous substances excite the sensory endings of fibers of the trigeminal nerve, which are distributed in the mucosa of the nose and which serve as nonspecific, common chemoreceptors. The receptors of the trigeminal nerve are especially important in the complex perception of the odors of irritants, such as acetic acid and ammonia.

The number of odorous substances is enormous, yet the odor of each one is unique: no two chemical compounds have an exactly identical odor. Several classifications of odors have been proposed, but none have been generally accepted. Compounds that differ widely in structure and compounds that are closely related structurally can have similar odors. To a large extent, the connection between odor and chemical structure has not been elucidated. Nevertheless, in practical work with odoriferous substances a number of empirical rules are used that sometimes make it possible to base a prediction of the character of the odor of a compound on the compound’s structure.

The mechanism of the primary interaction of a molecule of an odorous substance with the cell of an olfactory receptor has not been sufficiently studied. More research, usually involving measurement of bioelectric potentials, has been done on the subsequent stages in the transmission of a signal within the olfactory system. The surface of olfactory epithelium becomes more electronegative relative to the surrounding tissue in response to contact with an odorous substance. A mechanically produced diagram of this electrical reaction is called an electroolfacto-gram; it summarizes the individual responses of a large number of individual receptor cells. Contact with odorous substances results in a shift in the electrical potential of the plasma membrane of each receptor cell; this shift gives rise to nerve impulses or to a change in the impulses’ frequency. Usually, receptors respond by increasing the frequency of impulses; the degree of this change is a reflection of the degree of sensitivity of various receptor cells to different odorous substances. Some substances can produce a decrease in the frequency of impulses, and a given receptor may be completely insensitive to certain odorous substances. Receptors vary in their sensitivity to a given odorous substance. The composite perception that is effected by the many types of olfactory receptors is the basis for differentiating a large number of odors. The impulse responses of the neurons in the olfactory centers of the brain are differentiated on a similar basis.

In the antennae of insects, two types of electrical potential can be recorded that result from stimulation by odorous substances: a slow electroantennogram, which is analogous to the electrool-factogram, and nerve impulses from individual receptors. In many insects, certain highly specialized receptors have been found to react to only a single substance, such as a sex phero-mone. These receptors are in contrast to the other type of insect olfactory receptors, which resemble the vertebrate type in their broad range of perceived substances and their low degree of selectivity.

Olfactory sensitivity decreases in elderly persons. On the other hand, during pregnancy the degree of olfactory sensitivity may become uncomfortably acute. Olfaction decreases or completely disappears when the mucosa of the nose are congested or atrophied. Olfactory impairment is especially marked in cases of ozena or with tumors or traumas of certain regions of the brain. Disturbances of olfaction include hyperosmia—the increased sensitivity to odor—and olfactory hallucinations with certain mental diseases, in which nonexistent, usually unpleasant, odors are perceived. Hyposmia is the abnormally decreased sensitivity to odors, while anosmia is the total loss of olfactory sensitivity. Disturbances in the recognition of odors can also occur. Olfactory disturbances are treated by eliminating the underlying cause.

REFERENCES

Bronshtein, A. I. Vkus i obonianie. Moscow-Leningrad, 1950.
Minor, A. V. “Fiziologiia obonianiia.” In Fiziologiia sensornykh sistem, part 2. Moscow-Leningrad, 1972.
Moncrieff, R. W. The Chemical Senses, 3rd ed. London, 1967.
Ottoson, D., and G. M. Shepherd. “Experiments and Concepts in Olfactory Physiology.” In Progress in Brain Research, 1967, vol. 23.
Taste and Smell in Vertebrates. (Ciba Foundation Symposium.) London, 1970.
Olfaction and Taste, vols. 1–4 (proceedings of an international symposium.) Oxford, 1963–72.

A. V. MINOR

olfaction

[äl′fak·shən]
(physiology)
The function of smelling.
The sense of smell.
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